The present invention relates in general to an actuation unit for controlling the angular position of a control member, in particular a control member used in a vehicle transmission, for example for controlling a locking device for locking the vehicle transmission when the vehicle is parked (the so called park-lock device).
Document WO 02/21022 A1 discloses an actuation unit for a vehicle transmission comprising a control member, in particular a driving shaft, a primary motor, in particular an electric motor, a transmission mechanism which is interposed between the primary motor and the control member to transmit to the control member the motion generated by the primary motor and comprises two transmission members arranged to engage with each other, a spring for normally keeping the two transmission members disengaged from each other, and an electromagnet for moving one of the transmission members towards the other one to engage it. Such a known solution is affected by the problem that in case of interruption in the electric power supply the two transmission members are automatically disengaged from each other by virtue of the elastic force generated by the spring. In case of application of the actuation unit for controlling the park lock device of a vehicle, this means that in case of interruption in the electric power supply the vehicle is automatically locked in the parked condition, even though this is unnecessary and/or undesired.